Army veteran Brad Schwarz with his service dog, Panzer. Schwarz has suffered from PTSD since returning from a 2008 tour in Iraq. (Photo: Scott Olson/Getty Images)

The Department of Veterans Affairs will pay for service dogs assigned to veterans with impaired vision, hearing or mobility, but it will not cover the cost of dogs assigned for mental disabilities like post-traumatic stress disorder, according to new federal regulations.

Many dogs provide support to veterans suffering from PTSD, but although more veterans are being diagnosed with the anxiety disorder, the VA says there's not enough evidence that these dogs help with the symptoms of combat-related disabilities. A new Army policy has already made it more difficult for soldiers to obtain service dogs and keep them on Army bases.

There’s plenty of anecdotal evidence that service dogs help veterans cope with PTSD, but research lags and the VA is skeptical.

"VA has not yet been able to determine that these dogs provide a medical benefit to veterans with mental illness. Until such a determination can be made, VA cannot justify providing benefits for mental health service dogs," the department said.

There have been no double-blind, randomized controlled trials on the benefits of service dog and PTSD patients, and there are no widely accepted standards for training dogs to alleviate PTSD symptoms.

Researchers at the James A. Haley Veterans Hospital in Tampa, Fla., are conducting the first study to look at benefits of pairing veterans with PTSD with specially trained dogs. Congress recommended the three-year study, permitting the Department of Veterans Affairs to match as many as 200 veterans with dogs, but only 17 participants are currently enrolled.

Three service dog organizations partnered with the hospital to conduct the study, but Guardian Angels Medical Service Dogs (GAMSD) is the only association still involved in the research. The organization trains PTSD service dogs to perform such tasks as awakening clients from nightmares and reminding them to take medication.

"The results are very immediate. They’re very quick. It’s not a cure, but they are able to manage their challenges much better than they have in years,” Borden told NBC News.

Demand for PTSD service dogs is high, according to Borden, who says that most recipients spend four years on her organization’s waiting list.

Determining the need for service dogs

It’s estimated that 13 to 20 percent of the more than 2.6 million Americans who served in Iraq or Afghanistan since 2001 have or could develop PTSD.

But unlike service dogs for people with more obvious physical disabilities, there can sometimes be confusion over who can have a dog accompany them into certain places. The American Disabilities Act requires businesses to allow people with disabilities to enter with service animals, but dogs whose sole function is to provide comfort or emotional support do not qualify as service animals under the ADA. However, according to the ADA, dogs that calm a person with PTSD during an anxiety attack are considered a reasonable modification to ADA policies.

Service dogs assist disabled people with specific tasks like opening doors and pulling wheelchairs. Therapy dogs, on the other hand, provide comfort, motivation and emotional support. With proper documentation they can often be taken onto planes and other spaces where animals aren’t usually allowed.

Although PTSD service dogs are trained to respond to certain cues, such as nudging an owner into a petting session if he exhibits panic attack symptoms, and to perform tasks like reminding him to take medications, some people are skeptical of the idea that a dog can assist with a so-called "invisible" disability.

However, there’s evidence that interacting with animals produces biochemical changes in some people’s brains.

Research shows that when people focus on petting a dog, it can increase oxytocin, a chemical that quiets the brain’s fear response. Caring for a pet also helps people become more secure and self-sufficient, according to Hal Herzog, a psychology professor at Western Carolina University.

Training service dogs can also be a form of therapy, according to Rick Yount, founder of Warrior Canine Connection, an organization that has PTSD patients train service dogs. After completing a 2008 training program at a veteran’s hospital, many participants reported lower stress levels, decreased depression, better impulse control and improved sleep.

Yount says that it might be most effective for veterans with PTSD to train a service dog before receiving one themselves.

"They have to convince the dog the world is a safe place, rather than letting the dog prove to them that the world is a safe place,” he told MSNBC.